https://greekinter.net/margin-call-%ce%bf-%ce%a0%ce%ac%ce%bd%ce%bf%cf%82-%ce%9a%ce%b1%ce%bc%ce%bc%ce%ad%ce%bd%ce%bf%cf%82-%cf%83%ce%b5-%ce%ba%ce%b1%ce%bb%cf%8d%cf%84%ce%b5%cf%81%ce%b7-%ce%b5%ce%ba%cf%84%ce%ad%ce%bb%ce%b5/